For modeling particles-gas reactions in terms of the morphology of nascent solid reaction products, a technique using the environmental scanning electron microscope (ESEM) was developed to monitor the in situ oxidation of iron. Experimental conditions of the oxidation experiments were selected as the temperature range of 500-750 degrees C and the oxygen partial pressure range 10(-4)-45 Pa. The evolution of nucleation and growth of oxide whiskers or grains on the surface of iron specimens under the ESEM was recorded. The characteristics of some specimens were further examined after oxidation by post-analysis. Three sets of experimental results and other relative micrographs are shown. In the light of the surface morphology and textures of the oxide growths in the micrographs, their growth mechanism and reaction kinetics are discussed. Finally, a plausible physical model for oxidation of iron is suggested.